In the art there is currently interest in using smectite clays as catalysts. It is known that the unique structure permits modifications which provide useful catalyst properties.
In an article titled "Catalysis: Selective Developments", Chem. Systems Report 84-3, 239-249, at Section 3.4320, some of the unusual properties of smectite clays which make them of interest as catalysts are discussed. These compositions are layered and exhibit a 2:1 relationship between tetrahedral and octahedral sites. In addition, the combination of cation exchange, intercalation and the fact that the distance between the layers can be adjusted provide interesting possibilities. In addition, the cations on the interlamellar surfaces (Ca.sup.+2 or Na.sup.+) can be replaced with almost any cation desired by simple ion-exchange techniques.
Art is available which focuses on how various factors affect clay catalysts. In an article titled "Pillared Clays As Catalysts" in Catal. Rev. Sci. Eng., 30(3), 457-499 (1988) there is a discussion of factors affecting the properties of pillared clays, including methods by which the thermal stability can be improved in the range from about 480.degree. C. to about 800.degree. C. It was also found that the method used to dry the flocculated clay appeared to be more important in determining the porosity of the final product than the choice of pillaring agent or the clay layer charge. For example, adsorption is one property which varies with drying method. Also, certain chemicals such as Al.sub.2 O.sub.3 can be fixed on the clays in order to increase surface area by changing the pore size distribution and increasing thermal stability.
The same article also discusses the acidity of pillared clays as well as ways in which different treatments affect the Lewis or Bronsted sites to a varying extent. It appears to be necessary to invoke the presence of several types of acid sites, because pillared clays do not necessarily function in the same manner as zeolites.
There is a discussion of clay mineral catalysts, including acidic montmorillonite clay catalysts in "Progress in Inorganic Chemistry", Vol. 35, p. 41 (1987). The process of pillaring this type of catalyst is discussed. Pillaring can convert a clay lamellar solid from a material that survives no more than a few hundred degrees centigrade heating before collapsing, into a more heat resistant two dimensional zeolite-type material that can survive heat treatment in moist atmospheres well above 500.degree. C.
In U.S. Pat. Nos. 4,176,090 (1979) and 4,248,739 (1981) Vaughan et al. disclose the addition of a silicate to an aluminum chlorhydral solution to substantially increase the surface area of clays. Some of the best hydrothermal stability reported to date in the art concerns clay intercalated by hydroxy silica-aluminum ions.
It is known in the art that suitably modified smectite clays can be very selective catalysts for a wide range of organic reactions and that they can act as Bronsted and Lewis acids. This is discussed in "Clays as Selective Catalysts in Organic Synthesis", J. M. Adams and K. Martin, J. of Inclusion Phenomena, 5, 663 (1987). After surveying the Bronsted acid activity of such clays, Adams et al. found that with cation-exchanged clays, the reactions proceeded below 100.degree. C. provided they involve tertiary or allylic carbocation intermediates. At 150.degree.-180 C. reactions involving primary and secondary carbocations are possible.
The same author discusses the fact that potential Lewis acid centers exist in smectite clays. Al.sup.3+ and Fe.sup.3+ ions are normally associated with the octahedral sheets of the montmorillonite. At page 668 there is discussed the reaction of alcohols with isobutene in the presence of montmorillonite catalysts. This reaction gives high yields of the tertiary ether at low temperatures according to the equation: ##STR1## where the rate of reaction was found to be proportional to the isobutene concentration. A plausible reaction scheme is as follows: ##STR2## The rate determining step appears to be the protonation of the alkene. Apparently solvent effects can be great, with the use of 1,4-dioxane increasing the rate six-fold and promoting miscibility of all reagents. In contrast, the reaction of alcohols with linear alk-1-enes is slow and gives mixture of alk-2-yl and alk-3-yl ethers.
The use of Group IV Metal Phosphates is discussed in "Recent Advances in Pillared Clays and Group IV Metal Phosphates", A. Clearfield, Surface Organometallic Chemistry; Molecular Approaches to Surface Catlaysis, 231, 271 (1988). It is known that smectite clays swell with water and are able to exchange large cations such as [Al.sub.3 O.sub.4 (OH).sub.24 --12H.sub.2 O].sup.7+ and [Zr(OH).sub.2.4H.sub.2 O].sub.4.sup.8+. These large cations can then act as pillars to prop open the clay layers. This property allows for the creation of catalyst materials with pores larger than those found in zeolites. In particular there is disclosed the potential use of the layered compound .alpha.-zirconium phosphate which is pillared by cross-linking the layers with aryl diphosphonic acid. Where the clay is pillared the pillar spacing apparently is determined by the radius of the ingoing cation.
Smectites characteristically have alumina octahedra sandwiched between layers of silicate tetrahedra. In smectites, substitutions of Mg.sup.2+ for Al.sup.+3 and Li.sup.+ for Mg.sup.2+ can take place in the octahedral site or M.sup.3+ ions can substitute for Si.sup.4+ in the tetrahedral site.
In J. Am. Chem. Soc., 101. 6891 (1979) Pinnavaia et al. pointed out that the rapid tumbling of simple intercalated ions, such as hydrated Cu.sup.2+ and Mn.sup.2+ and movement of free solvent in the clay interlayers indicate the possibility of carrying out metal complex catalyzed reactions in the intra-crystal space of said minerals.
No art has been found suggesting a C-5 raffinate stream could be reacted with a low molecular weight alkanol to synthesize desirable commercial products such as TAME. Synthesis of a product such as tertiary amyl methyl ether by rapid conversion of methanol and a C-5 olefin over an acidic clay would have significant commercial value. Such a process would be particularly efficient if TAME were manufactured as a primary or secondary product from typical C-5 raffinate streams from the production of MTBE, butadiene and light olefins. Furthermore, the acidic clay catalyst described is particularly efficient and thermally stable.